How does La Niña affect tropical cyclones?

Eastern Queensland is one of the wetter regions in Australia due to its proximity to the world’s most intensive rainfall band, located in the Western Pacific.

The rain band, which is powered by the warmest ocean surface temperature on the globe, moves eastward during El Niño, tending to reduce eastern Queensland rainfall.

The rain band moves westward during La Niña conditions, leading to increased rainfall in eastern Queensland.

Many of the wettest years for the region have occurred in La Niña years including 1918 1956, 1971, and 1974.

The Queensland coast experiences three to four times as many cyclones during La Niña years as during El Niño years. The current La Niña is one of the strongest on record.

The La Niña-rainfall relationship oscillates over several decades, in tandem with a longer-lasting feature called the Interdecadal Pacific Oscillation (IPO).

The IPO has a somewhat similar pattern to the El Niño-La Niña cycle but on a longer time scale.

The IPO has been in a ‘positive phase’ similar to El Niño conditions since the late 1970s, limiting the rainfall that La Niña brings to Queensland by disabling a major rain-generating mechanism and being largely responsible for the recent drought in south-east Queensland.

The number of tropical cyclones also decreases during a positive IPO phase.

The IPO appears to have moved to a phase similar to La Niña during 2010. T

his may increase the impact of La Niña conditions on Queensland rainfall. The number of tropical cyclones also increases during such a ‘negative phase’ of the IPO.

It is difficult to discern long-term variations in the characteristics of tropical cyclones because of limitations of our long-term data, especially prior to the availability of satellite data since the late 1970s.

No significant global trends have been detected in the frequency of tropical cyclones to date, and no significant trends in the total numbers of tropical cyclones, or in the occurrence of the most intense tropical cyclone, have been found in the Australian region.

"No significant global trends have been detected in the frequency of tropical cyclones to date ..."

There is evidence of changes in other aspects of extreme weather patterns.

There have been fewer cold days and nights and more hot days, hot nights and heatwaves globally in the past 50 years. Heavy rainfall events have increased in frequency over most areas of the globe.

Precipitation has increased significantly since 1900 over eastern parts of the Americas, northern Europe, parts of Asia and north-west Australia.

Reduced precipitation has occurred in central and southern Africa, the Mediterranean, and parts of southern Asia.

Eastern and south-western Australia have become significantly drier since 1950, while north-western Australia has become wetter.

The interaction of short-term and long-term variations can either reduce or worsen the impacts of extreme events, making it more difficult to pinpoint the causes of local climate trends or specific weather events.

How do climate projections suggest the pattern of tropical cyclones and extreme weather events will change?

The effects of climate change will be superimposed on natural climate variability, leading to changes in the frequency and intensity of extreme weather events.

The underlying warming trend of oceans around the world, which is linked to human-induced climate change, will tend to increase the risk of extreme rainfall events in the short to medium term.

This is because a warmer ocean tends to increase the amount of moisture that gets transported from the ocean to the atmosphere, and a warmer atmosphere can hold more moisture and so have greater potential for intense rainfall events.

Tropical cyclone days are projected to decrease in frequency in the Australian region, but it is expected that a greater percentage of total cyclones will be in the stronger categories. That is, we may have fewer cyclones but the ones we do have will be stronger.

Cyclones and other low pressure systems can cause oceanic storm surges. These are projected to become larger, leading to more coastal flooding when superimposed on sea-level rise.

Rising average sea level will mean that storm surge effects on our coasts will increase in frequency even if the height of storm surges above mean sea level does not change.

Heatwaves and fires are likely to occur more often and with greater intensity in future decades. Frosts and snow-storms are likely to occur less often.

Days with heavy rainfall are projected to become more intense over most areas of Australia.

The number of days with large hail is projected to increase along the east coast from Fraser Island to Tasmania and decrease along the southern coast of Australia.

What is the impact of combined increases in sea-level rise, severe storms and coastal population?

The combined influence of increasing sea-level rise and extreme weather will result in an increase in storm-surge related flood heights and frequency.

An increase in Australia's population in coastal areas will mean more people and property will be exposed to the impacts of extreme weather events.

CSIRO research suggests that a rise in sea level of around 0.2 m and a 10 per cent increase in cyclone intensity could double the area of Cairns affected by a present-day 1-in-100 year storm-tide.

This does not account for rainfall and likely changes in the intensity of rainfall accompanying tropical cyclones, so the impact of climate change may be even greater than this.

Results also suggest damage costs associated with flooding in southern Queensland and northern NSW will increase by 50 per cent if sea level rises by 20cm and more than double if sea level increases by 40cm.

These changes will mean flooding over an area much larger than has been historically affected. What isn't a flood prone area now may become one in the future.